The present invention relates to an improved process which cures silicone compositions by way of a precious metal-catalyzed reaction of silicon-bonded radicals and results in an improved bath life. The present invention further relates to an improved process for increasing the bath life and/or cure time of heat curable silicone compositions.
Processes describing a platinum group metal-containing catalyst being inhibited in its cure-promoting activity at room temperature by the presence of a catalyst inhibitor are well known in the organosilicon art and need no detailed discussion herein.
The use of maleates and fumarates in inhibitor systems is well known in the art. There are several literature sources describing their preparation and uses.
The maleates have been found to be particularly effective for increasing the room temperature bath life, i.e. work time, of solventless coating organosilicon compositions which cure by way of a platinum group metal-catalyzed reaction. However, the heating time and/or temperature needed to cure in these maleate-inhibited systems is sometimes excessive. When one attempts to decrease the cure time and/or temperature of silicone compositions to a commercially desirable interval by using less maleate and/or more catalyst in these inhibitor systems the bath life is frequently decreased to a commercially undesirable interval.
The fumarate inhibitor systems have been found to allow a cure of a solventless coating organosilicon compositions which cure by way of a platinum group metal-catalyzed reaction to take place at a suitable heating time and/or temperature. However, the bath life of such a composition, as measured by gel time at room temperature, is not as long as desired. When one attempts to increase the bath life of these compositions by increasing the amount of fumarate and/or decreasing the amount of catalyst in the fumarate inhibitor systems the cure time and/or temperature increases.
This problem of increased cure time and/or cure temperature with increased bath life in an inhibited platinum group metal-catalyzed system is of particular significance for applications where the organosilicon composition is used to rapidly coat a substrate over a long period of time. In such a process a long bath life coupled with a short cure time, preferably at low-to-moderate temperature, is an essential property of the coating composition.
In the coating arts, such as the paper coating art, the coating composition that is used to coat a substrate should not cure to the extent that its viscosity has increased substantially before it has been applied to the substrate; however, it should rapidly cure thereafter, preferably with only a moderate amount of added energy. Typically this means that the coating compositions preferably should not gel for as long as eight hours but should cure rapidly at moderately increased temperature to such an extent that the coated substrate can be further processed, if desired, without damaging the coating. In addition, the cure time of the composition at a given cure temperature desirably should remain substantially constant as the bath ages.
In the preparation of laminates comprising a peelable release paper bearing a cured coating and an adhesive film releasably adhered thereto, such as a stick-on label, one of two processes is normally used. In one process, the off-line process, the silicone composition is coated on paper and cured; then, at a later time, an adhesive film is applied to the cured silicone coating. In the other process, the in-line process, the silicone composition is applied to paper and cured and the adhesive is then immediately coated on the cured silicone coating. While the in-line process is generally more efficient and would normally be more advantageous, it has been found that some adhesives bond, i.e. weld, to the silicone coated paper if the adhesive is applied to the silicone coated paper within a short time after the silicone composition has been cured. It is believed that residual reactivity in the cured silicone coating is responsible for the welding of the silicone and adhesive in the in-line process.
While the art has proposed and provided some solutions for the welding problem there is a need for further improvements in a release-coating process for in-line lamination of adhesives which cures at lower temperature and/or has a longer bath life.
There is also interest in applying silicone release coatings to substrates, such as polyethylene sheets, which are less stable at the elevated temperatures used to cure the silicone release coatings of the art. Consequently, there is a need for silicone coatings which cure at reduced temperatures. Of course, silicone coatings which have lower curing temperatures or faster curing rates are also desired for energy conservation and for more efficient production processes.
Several inhibitor systems have been used in the art to produce coating compositions which have a lower bath life and/or cure time. For example, U.S. Pat. No. 4,256,870 issued Mar. 17, 1981 to Eckberg teaches a method of producing a coating composition that has an improved bath life by mixing in any order a polysiloxane base polymer, a methylhydrogen crosslinking agent, a platinum catalyst, and diallylmaleate is added as an inhibitor to effectively retard the hydrosilation addition cure reaction of the composition at ambient temperature, but which does not retard the cure at elevated temperature.
Eckberg, U.S. Pat. No. 4,262,107 issued Apr. 14, 1981 teaches a one-part or two-part inhibitor system which produces a paper release coating composition with an improved bath life and cure time by mixing in any order a silanol polymer, a methylhydrogen crosslinking agent, a rhodium catalyst, and a low molecular weight silanol endstopped diorganopolysiloxane alone or in combination with a diallylmaleate as an inhibitor.
Lo et al., U.S. Pat. No. 4,562,096 issued Dec. 31, 1985 teaches a one-part inhibitor system that produces a release coating with an improved cure time by mixing in any order a vinyl functional polysiloxane base polymer, an organohydrogenpolysiloxane crosslinker, a platinum catalyst, and a hydrocarbonoxyalkyl maleate as an inhibitor.
Eckberg, U.S. Pat. No. 4,476,166 issued Oct. 9, 1984 teaches a two-part inhibitor system that produces a solventless silicone release coating with improved bath life and cure time by mixing in any order an olefinorganopolysiloxane, an organohydrogenpolysiloxane as a crosslinking agent, a platinum catalyst, and a blend of diallylmaleate and vinyl acetate as an inhibitor.
Fumarate inhibitor systems have also been described in the art. For example Lo, U.S. Pat. No. 4,774,111 teaches a process for improving the bath life and cure time of heat-curable silicone compositions by mixing in any order an organosiloxane base polymer, an organohydrogenpolysiloxane crosslinker, a platinum catalyst, and a diorgano fumarate cure control additive as an inhibitor. The most recent inhibitor system that has been described in the art is in U.S. application for patent, Ser. No. 431,352, filing date Nov. 3, 1989, and assigned to the same assignee as this present application, which teaches a process for producing a curable organosilicon composition comprising mixing, in any order, a component having silicon- bonded hydrogen atoms, a component having silicon bonded radicals reactive therewith, a curing catalyst, a catalyst- inhibitor, and a bath life extender. In other words the patentees of the above filed application teach a five component system with no critical order of mixing these components. In contrast, the present invention teaches a four component system with an optional bath life extender component that, with a specific order of mixing, unexpectedly produces a composition that has a longer bath life and/or shorter cure time than the comparable processes described in the art.
The inhibitor systems taught by the existing art do not describe a process which greatly reduces the cure time and/or significantly increases the bath life of heat-curable silicone compositions by utilizing a specific order of mixing the components to achieve the above unexpected results.